Humans are able to report the number of objects they see rapidly and accurately up to about four objects. Beyond four, accuracy begins to diminish and response times increase with each added item. The ability to quickly and correctly report small amounts is called subitizing. It has been suggested that subitizing may rely on the underlying use of a parallel, preattentive mechanism for individuation with a capacity limit of about four.

While fixating on the central cross in the animation above, you should have no difficulty immediately judging the number of targets which flash before you.

What sort of processing underlies this ability? How are candidate objects individuated and enumerated so quickly by the visual system? To begin answering these questions, we’re exploring the impact of occlusions on human subitizing performance. By studying the impact of partial occlusion on subitization, we will gain insight into the mechanisms behind this process. How much processing occurs in the moment before a numerosity judgment? Can separate visual patches on either side of an occluder be resolved as one object without incurring an extra cost in terms of reaction time or accuracy?